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Re: EXTENT BOUNDARIES

To: Les Oxley <les@xxxxxxxxx>
Subject: Re: EXTENT BOUNDARIES
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Fri, 19 Jan 2007 22:34:37 -0600
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Les Oxley wrote:

Hello,

We are looking into running XFS on a 3TB FLASH MEMORY MODULE. We have a question regarding the extent boundaries.
See the attached PowerPoint drawing, xfs.ppt We are running Linux.
Our media is 3 million contiguous 4KB blocks. We would like to define an extent size of 1MB and this tracks the erasure block size of the flash memory, and that greatly improves perfomance. We are trying to understand where XFS places the extent boundaries with reference to the contiguous block sequence. Is this deterministic as indicated in the drawing ? That is, are the extent boundaries on 256 block boundaries.

Any help would be greatly appreciated.

Les Oxley
Ampex Corporation
Redwood City
California.

extents by definition land on filesystem block boundaries, and can in general be any number of filesystem blocks, starting & ending most anywhere on the block device.

If you wish to always allocate in 1m chunks, you might consider using the xfs realtime subvolume, see the extsize description in the mkfs.xfs man page. I'm not sure how much buffered IO to the realtime subvol has been tested; pretty sure it works at this point, though the sgi guys will correct me if I'm wrong... it's not exactly the normal mode of operation.

Using the realtime subvol, however, all your file -metadata- will still be allocated on the main data volume, in much smaller pieces.

-Eric


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